"I feel uneasy about everything."

- Yūya

Yūya Mochizuki (望月 優矢, Mochizuki Yūya) is a student of 9th Grade Class 3 in 1998 and one of the older members of the Art Club at Yomiyama North Middle School. He has an older half-sister named Tomoka Inose who works at a café.

In the anime, Yūya is a thin young man of below average height with a pale complexion. He has long, light brown hair, which frames his face, and green eyes. As a student, he is seen wearing the standard male uniform, except that he does not wear the Gakuran.

He was also seen wearing a pink shirt during the trip to the beach, and a purple T-shirt with an image of a clock on a branch (a clear reference to the painting known as "The Persistence of Memory" made by the artist Salvador Dalí in 1931).

In the manga, Yūya has light hair and black eyes. He also has a more feminine look.

Kōichi comments, while describing their first meeting in the novel, that if Yūya were to "dress in drag" and go to Shibuya (a seedy district of Tokyo), he'd probably get "picked up" by someone mistaking him for a girl.

In the film, Yūya has black hair and black eyes.

Yūya is calm, innocent, patient, quiet around strangers, and a bit shy, but is cheerful and sociable around his friends. He is good friends with Naoya Teshigawara and becomes Kōichi Sakakibara's first friend from Yomiyama North Middle School; he was also fairly close to Ikuo Takabayashi and was horrified when the other boy died.

Yūya seems to be one of the few students of Class 3-3 everyone likes (a frustrated Izumi Akazawa yells at him once, but mostly out of being angry with herself, and even Takako Sugiura only seems to find him mildly irritating, as he's one of the few people she behaves playfully with). He was a fixture of the art club as one of their senpai (the other being Mei Misaki); he uses this to her disadvantage in an attempt to keep her isolated by luring the rest of the club away when Mei and Kōichi attempt to visit.

Incredibly shy, Yūya confides to Kōichi that he finds everything uncomfortable. His feminine appearance is one source of this anxiety, especially concerning Tomohiko Kazami's sisters. Though he never admits it to her, Yūya has a strong infatuation for Reiko Mikami, more so than Naoya, who merely finds her attractive in an abstract sense. Yūya is strangely at his most courageous around her, going out of his way to be around her when the group went to the beach, and generally acting far more outgoing. He is also one of the few people, alongside Izumi and Mei, to figure out that Reiko was also Ms. Mikami.

Yūya is easily pressured into doing things he'd rather not, such as allowing Tomohiko's sisters to openly discuss dressing him as a girl, or meekly accepting Kōichi, Izumi, and Takako burying him in the sand at the beach, even submitting to Takako tickling him. Izumi, unfortunately, is aware of this and uses it against him, pressuring an isolated Yūya into playing the tape he'd recovered from Katsumi Matsunaga, an act which has tragic consequences for Class 3-3. He can be very loyal to his friends and refuses to abandon them, such as when he dragged Naoya away from a crazed Keiko Numata, or when he defended Mei from Izumi at dinner.

As Naoya's best friend, Yūya was one of the students who welcomed Kōichi Sakakibara on his first day in Class 3-3. An extremely friendly boy, Yūya, along with Tomohiko Kazami and Naoya Teshigawara, showed Kōichi around the school at lunch. On the other hand, he was unwilling to explain the curse to Kōichi.

In "Build Limbs", Yūya and Ikuo Takabayashi are invited by Kōichi to walk home. The trio stop beside the river, where Ikuo decided to tell Kōichi about the curse. Unfortunately, before he could do so, he had a fatal heart attack. Yūya was unable to save his friend. 's mangled corpse in horror.]]A few days later, Yūya was one of the students who attended the big countermeasures meeting wherein Izumi decreed the class would, from now on, ignore both Mei and Kōichi. Yūya was unable to keep to the class rules, however, and feeling guilty, left a letter underneath Kōichi's desk, stating Kōichi should talk to Mei, as she'd be able to explain the rest.

In "Sphere Joint", Yūya participates in the meeting at the Inoya Café (which his family owned and at which his half-sister worked), organized by Naoya, along with Izumi and Kōichi.

In "Hair Stand", Yūya went with Reiko Mikami, Kōichi, Naoya, and Izumi to the hotel where Katsumi Matsunaga works and lives. Unable to meet him, however, the kids decided to wait at the beach and spend the day relaxing, assuming they were safe from the curse. Yūya was buried in the sand by Izumi and Kōichi, while Takako Sugiura tickled his feet to keep him from resisting, while also getting revenge for him and Naoya tossing her into the ocean earlier. Naoya decided to have a "contest"; fishing for lunch ingredients. Yūya was in the non-existent team with Mei and Kōichi, much to his irritation. Though he had a fun day with his friends at the beach, a pall was cast on things when Junta Nakao started to drown while trying to recover Takako's beach ball. Yūya was the only boy who remained on the beach, as Naoya, Kōichi, and Katsumi rushed to help him. They were, unfortunately, too late, and Junta was hit by a speedboat and eviscerated, much to the group's horror.

In "Body Paint", Yūya meets up with Kōichi and Naoya to look for the item that Katsumi left in the old classroom. They find a recording tape, which Naoya accidentally broke later on. Yūya offers to repair the tape himself.

In "Glass Eye", Yūya repaired the tape and heard the most important part along with Mei, Naoya, and Kōichi. Also, he took Mei's side when Izumi confronted her about the deaths.

In "Makeup", after knowing that there was a killer around, Yūya suggested looking for Ms. Mikami first. He and Naoya went around to check, and when they reached a room. They found Keiko Numata in the room with a blood-stained cleaver. She attempts to attack them, and during the process, the cleaver cuts Naoya's leg. Yūya went to help Naoya escape from Keiko, but she followed them along the hallway. Also, he showed the tape to Takako Sugiura and Izumi, which had tragic consequences for both. After a fire broke out at the inn, Kōichi and Naoya had to warn their classmates of the danger. Unfortunately, when trying to find Ms. Mikami, they instead encountered a deranged Keiko, who'd already nearly killed Takako and Manabu Maejima. The two were forced to abandon their mission as she began to pursue them through the burning inn.

In "Stand By Oneself", Yūya pushes Naoya out of a window while running from Keiko. He was about to jump out as well when Keiko grabbed his belt and started pulling him back up towards her. Naoya grabbed him and pulled back, sending Keiko and Yūya crashing to the ground. Keiko attempts to kill Naoya, but they're saved by Tatsuji Chibiki, who kills Keiko and brings the boys to his car, where he tends to Naoya's leg.

The anime ends with Yūya and Naoya putting a tape they recorded about how to stop the calamity into the locker where they found Katsumi's original one.

Yūya and Kōichi became friends quickly after they met. They talked about art among other things. He was present when Ikuo died; before he died, Kōichi asked him for the class roster - after ignoring Kōichi, he slips a copy into his bag and asked for forgiveness as he did not want to pretend that he doesn't exist. They were able to make amends and stayed friends.

In the manga and anime, Yūya and Ikuo had a good friendship. When Ikuo suffered a heart attack, Yūya immediately ran to him and tried to help. After his death, Yūya became a bit depressed. In the manga, Yūya only mentioned Ikuo's name, and only got the information about his death by a phone call. In the anime, their friendship was shown more.

Yūya and Mei barely interacted throughout the manga and anime. The two were paired, along with Kōichi, by Naoya for the catching contest. The two didn't interact much, but were on friendly terms. In Episode 10, Yūya defended her from Izumi. However, he doesn't like openly associating with her, even going so far as to lure the rest of the art club away from Mei when she and Kōichi attempted to visit them, and indignantly exclaiming "I exist!" when Naoya put him on the "nonexistent" catching team with Mei and Kōichi.

Naoya and Yūya are friends and usually do things together. Throughout the anime and manga, they are usually together. When Keiko went insane and wounded Naoya, Yūya aided and saved him, and in return, Naoya saved him as Keiko tried to attack him. After the Calamity ended in 1998, they both created a tape and thoroughly explained what to do; they put the new tape in the locker where they found Katsumi's.

In the film, Naoya becomes convinced that Yūya is the Extra and attempts to kill him at the inn. He isn't successful, and upon being confronted by Tomohiko about his actions, Naoya fled, only to become entangled in a live electrical cable and be killed.

Yūya had a crush on Ms. Mikami. He spent time with Reiko when they went to the beach ("Hair Stand") and was ecstatic to ride in the Mikami's car next to her. Later, he and Naoya went looking for her, though instead they only encountered Keiko, who immediately tried to kill them. In the manga, it is made much clearer that he has a crush on her, as he visited Kōichi's house specifically to check on her at one point to make sure she was okay.

Tomohiko and Yūya are friendly, though not especially close in the anime. In the manga, where Tomohiko is a much more major character, the two are together more often, and Tomohiko's sisters have a crush on Yūya. At the end of the manga, Yūya and Tomohiko were together with Naoya at the hospital when he went to see Izumi, suggesting the three's friendship remained strong following the catastrophe.

In the film, Tomohiko was with the other boys when they learned how to stop the Calamity. He left Naoya and Yūya to explain things to Izumi and Takako, but warned Naoya not to do anything stupid. Unfortunately, Naoya didn't listen, and Tomohiko watched him attack Yūya. Later on, Tomohiko angrily confronted Naoya about what he'd done, suggesting he was deeply disturbed by his classmate's actions.

The two don't interact much, though Yūya seems to be one of the few people Takako doesn't find extremely irritating. Yūya and Naoya threw her in the ocean in "Hair Stand", and later on, when the group buried Yūya in the sand, Takako tickled his feet to make sure he didn't try to escape, suggesting at least some degree of familiarity. However, the two didn't interact again outside that episode, as Takako spiraled into depression following Junta's death, and Yūya focused on his friends' efforts to break the curse. However, when Yūya tried to bring the girls up to speed on his friends' progress by playing them Katsumi's tape, he inadvertently planted the seeds of Takako's psychotic breakdown.